The invention relates generally to reducing microstructural defects in line-of-sight deposited films and more particularly to the elimination of columnar growth defects in physical vapor deposited films by improved deposition methods.
Line-of-sight deposition and coating techniques are quite useful to industry. On a macroscopic scale such techniques provide the capability of uniformly applying a thin film or coating to a surface. Moreover, physical vapor deposition methods, such as sputtering, evaporation and similar line-of-sight deposition techniques, make possible the deposition of solid coatings directly from a vapor state. The resultant coatings have microscopic characteristics that are unobtainable by any other means. Physical vapor deposition methods, and especially sputtering, also enable deposition of films and coatings of a virtually infinite variety of materials. Such coatings can be amorphous or crystalline, metallic or non-metallic, and can be uniformly composed of non-equilibrium combinations of elements in proportions which ordinarily form, in equilibrium, a non-uniform composition or structure when deposited by other techniques.
In general, physical vapor deposition employs some mechanism to eject atoms of coating material from a source or target with sufficient energy to travel along a line-of-sight to the surface of a substrate to be deposited thereon. Physical vapor deposition includes sputtering, evaporative deposition, ion plating, and neutralized ion beam coating. It does not ordinarily include chemical vapor deposition, electroplating, or rapid solidification coating techniques. Ion plating is a variation of both sputtering and evaporative deposition which involves the ionization of atoms in the vapor followed by attraction of some portion of the ionized atoms to the substrate with an electric field. The principal characteristic of these techniques is that they utilize a line-of-sight access of some portion of the material source to the surface to be coated. The concept of line-of-sight access is broadened slightly in ion plating. That method modifies slightly the trajectory of ionized atoms of coating material to enable application of some material to portions of a substrate that are not on a true line-of-sight from the source. However, all of these techniques are essentially line-of-sight deposition methods whose coatings are influenced in generally the same way by line-of-sight defect-producing mechanisms. One such mechanism is geometrical shadowing, which produces columnar growth defects as hereafter described and shown.
Since sputtering is the most important of the presently known physical vapor deposition methods, and is representative of the other methods, the remainder of this discussion will concentrate on sputter deposition. However, the problems and principles discussed hereinafter are to be considered as equally applicable to all physical vapor deposition techniques and to other line-of-sight deposition methods as well.
Sputtered atoms which are emitted generally in the direction of the substrate are deposited as a film or coating on the surface of the substrate. If the substrate and target are aligned parallel planar plates, and the minimum angle of adatom incidence is large, then the entire coating will have a uniformly high quality. However, if the substrate is angled with respect to the target, is large, is wider than the target, or has a three-dimensional surface with portions angled from target, then at least a portion of the coating will be of poor quality. This problem is illustrated in greater detail in FIGS. 2, 5a to 5c, and 14a to 14f.
It has been determined experimentally that the angle of incidence of the net flux on the substrate surface strongly influences the quality of the resultant coating. Geometric shadowing was found to be a principal mechanism by which columnar growth defect structures separated by open boundaries are formed. These structures are generally associated with reduced corrosion resistance and other localized degradation of coating properties. The results of these studies are reported in the article "The Influence of Surface Topography and Angle of Adatom Incidence on Growth Structure in Sputtered Chromium," by J. W. Patten, presented in April, 1979 at the American Vacuum Society's International conference on Metallurgical Coatings, San Diego, Calif., and published in Thin Solid Films, Vol. 63, 1979, pages 121-129. Pertinent aspects of these results are discussed hereinafter with reference to FIGS. 5a to 5c and 14a to 14f.
It would be preferable if line-of-sight deposited coatings, and especially physical vapor deposited coatings could be formed without defects due to geometrical shadowing, and particularly without columnar growth structures and open leaders or boundaries between such structures. The open boundaries degrade the mechanical, electronic, and other physical properties of coatings and thus detract from their usefulness in engineering applications. For example, such coatings fail to protect the surface of the substrate from penetration of foreign substances, particularly corrosive fluids. They are also more susceptible to mechanical failure than coatings lacking such defects. The surfaces of such coatings also are often rough. These features are all highly disadvantageous for protective coatings applied to such substrates as marine gas turbine vanes and blades.
Several techniques have been tried to eliminate columnar growth defects from such coatings. One approach involves rotating the substrate as material is being deposited thereon. This technique results in a uniformly mediocre coating which still contains columnar growth defects. Another approach has been to try to manipulate the static geometrics of the target or the substrate or both so as to deposit uniformly at a right angle everywhere on the substrate, as disclosed in FIGS. 5 and 5a and U.S. Pat. No. 4,038,171 to Moss, et al. However, this method also does not satisfactorily eliminate defects due to geometrical shadowing.
Another technique involves heating the substrate after coating to increase the lateral thermal diffusion of material deposited thereon to "heal" the defects and thereby reduce the porosity of the coating. However, heating sufficiently to diffuse the materials laterally, for example, to a temperature of about 80% of the Kelvin melting point for a material such as sputtered copper alloy, allows the deposited materials to segregate into equilibrium crystallites of different phases. The hotter or the longer the heat treatment, the greater this tendency toward equilibrium. Phase segregation reduces both structural and compositional homogeneity of the entire coating, not only in those areas containing columnar growth defects, but in those regions having a high quality closed microstructure. Consequently, one of the principal purposes of physical vapor deposition, namely obtaining a non-equilibrium homogeneous coating structure and composition, is defeated. Another problem with heat treating is that it tends to increase grain size in the coating. The disadvantages of inhomogeneous structure or composition, or large grain size, are readily apparent to persons skilled in the coating art.
Another difficulty arising from heat treatment is the degradation of coating-to-substrate adherence. If the thermal expansion coefficient of the substrate and the coating are quite different, fracturing at their interface can occur. In addition, vertical diffusion of material away from the interface is likely to occur producing voids at the interface or, in some instances, brittle phases. Both of these mechanisms weaken the coating-to-substrate adherence.
A related technique involves coating at an elevated temperature so that sufficient lateral diffusion occurs as the coating is deposited to produce a dense coating. The same disadvantages as those described above apply.
Mechanical treatment of the coating, such as shot-peening, in combination with heat treatment allows somewhat lower temperatures to be used. However, shot-peening can also degrade coating-to-substrate adherence, particularly if the Young's modulus of the substrate differs substantially from that of the coating, by causing fracturing at the interface. In the case of very brittle coatings shot-peening without fracturing the coating is impossible.
Even combining deposition of a first material onto highly cleaned pin surfaces while rotating, followed by deposition of an overlayer of a different material and subsequent heat treatment, fails to eliminate defects due to geometrical shadowing, including columnar growth defects. Referring to FIG. 18, many voids or leaders remain, and some extend vertically through more than half of the thickness of the coating. After a portion of the coating wears away during use, such voids will be exposed.
Accordingly, there remains a need for a physical vapor deposition method which eliminates columnar growth defects without requiring mechanical or thermal treatments. For many purposes, it would, at least, be desirable to obtain a coating in which voids or leaders do not extend completely through the coating; that is, are limited to a fraction of the thickness of the coating. It would be even better if such voids or leaders were limited to about the height of the asperities which cause them. However, it would be most preferable to have a deposition method which would provide extremely high quality coatings which are essentially unaffected by geometrical shadowing.
A variety of sputtering methods have been proposed whose objectives are to obtain specific coating characteristics. For example, in U.S. Pat. No. 3,021,271, to G. Wehner it was proposed to use ion bombardment of the substrate to effect controlled resputtering of deposited material to maintain the overall rate of deposition below a predetermined critical value. The purpose was to grow monocrystalline coatings rather than the polycrystalline coatings having small crystallites which are formed by high rates of deposition. In U.S. Pat. No. 3,736,242, to N. Schwartz et al., resputtering was a means of controlling the crystalline phase structure and, thus, the resistivity and temperature coefficients of deposited films. In U.S. Pat. No. 4,036,723, to B. Schwartz et al., resputtering at different rates during deposition to avoid initial preferential etching of crystal grain boundaries in polycrystalline substrates and to thereby form a smooth insulative layer on a substrate. U.S. Pat. No. 4,038,171, to Moss et al., discloses a high deposition rate sputtering apparatus in which the substrate can be negatively biased during operation. Resputtering can thus be obtained in such apparatus if desired.
In each of the foregoing patents, the surfaces of the substrate and the source of sputtered material were parallel, planar and of approximately the same lateral dimensions. In such patents, substantially all of the material is deposited nearly perpendicularly to the substrate surface. Hence, the problem of geometric shadowing was not addressed in these patents.
U.S. Pat. No. 4,006,070 to King et al. discloses apparatus for sputtering metal oxide films on substrate surfaces of large lateral dimensions, such as a windscreen for a vehicle. The apparatus includes multiple, laterally spaced-apart sources of material which are reciprocated laterally along the substrate during deposition. The amplitude of reciprocation is sufficient to cause material to be deposited substantially uniformly over the entirety to the surface. However, it does not appear that King et al. addressed the problem of geometrical shadowing. Although the windscreens are curved, the sources are positioned along a parallel curve so that shadowing in the curved portions of the surface may be reduced. However, it appears that some geometrical shadowing is likely to occur on portions of the substrate below the spaces between the sources. Nevertheless, King et al. make no attempt to minimize the effect of geometrical shadowing on the resultant film. Defective regions of the coating are simply covered up as a result of reciprocation of the source assembly during deposition.
In our own prior work, described in ASME Gas Turbine Division Paper 74-GT-100, "Initial Work on the Application of Protective Coatings to Marine Gas Turbine Components by High Rate Sputtering", authored by E. D. McClanahan et al., Mar. 30--Apr. 4, 1974, and in 1977 Tokyo Joint Gas Turbine Congress Paper No. 64, "Recent Developments in the Application of High-Rate Sputtering Technology to the Formation of Hot Corrosion Resistant Metallic Coatings", authored by J. W. Patten et al., May 22-27, 1977, we experimented with several approaches to solving the problems of obtaining high quality coatings. The former paper discloses coatings on small planar surfaces, both in an as-sputtered condition and after heat treating. In some of the experiments, biasing of the substrate to -30 and -50 volts DC was tried and was found to have some effect on the coarseness of columnar grain structure. However, changes in deposition temperature had similar effects and the relative contributions of each parameter were not determined. The latter paper discloses both high- and low-integrity sputtered coatings on three-dimensional turbine components, both before and after heat treating, and includes coatings obtained by rotation of the substrate. However, no reference is made to biasing of the substrate. Nor does this paper teach the advantages of eliminating columnar growth defects due to geometrical shadowing by manipulating the method of deposition rather than resorting to post-deposition heat treatments. Finally, neither paper discloses the mechanism by which columnar growth defects are formed or a method for inhibiting their formation or propagation.